The present invention relates to the detection of breaks within a structure.
It applies particularly, although not exclusively, to the detection of breaks within a cable, such as a suspension bridge main suspension cable.
The main parts of a suspension bridge are recalled schematically in FIG. 1. This shows a suspension bridge comprising a deck 1 providing continuity of the carried way and the distribution of the forces. Hanger cables 2 support the deck and transmit the forces to the main suspension cables 3 to which they are securely attached by hanger cable collars. The main cables 3, which adopt a parabolic appearance, provide the support function. The forces can be broken down into a vertical reaction absorbed by the towers 5, and a tensile force transmitted by an anchor cable 4 securely attached to an anchor.
The integrity of a construction such as this relies in the ability of the main cables 3, of the hanger cables 2 and of the anchors to withstand the stresses resulting from the transfer of force, over an extended period of time.
These elements are thus the weak points of suspension bridges. Safety and endurance are therefore generally ensured by adopting suitable safety factors, especially as there is no redundant load path in such constructions.
The main suspension cables 3 usually consist of metal strands, generally made of steel, which are substantially parallel (although the strands are sometimes twisted). These strands are protected against corrosion by various means: heat treatment, chemical treatment, the application of paint, sheathing, etc.
However, it is impossible to rule out entirely the possibility of some of the strands that make up such main suspension cables breaking as a result, for example, of oxidation. This phenomenon is insidious because it mainly affects the internal strands around which water may infiltrate and stagnate without being eliminated by evaporation and without being immediately visible.
The substantially parallel configuration of the metal strands of the main suspension cables means that these strands rub together to a certain extent, thus limiting the extent to which any strand or strands that has or have broken retreat(s) away from a region surrounding the break point.
There then occurs what is known as re-anchorage, that is to say that, beyond this zone, the broken strands continue to contribute to the transmission of force and find themselves once again under stress. Only the break zone has a cross section that is reduced by the cross section of the broken strands and therefore a higher stress is exerted on the remaining strands in this zone. This may cause the remaining strands in the breakage zone to break if the permissible stress is exceeded. If the cause of the breakage of the initial strand or strands is still present in this zone, then this risk becomes more of a reality.
For these reasons, it is therefore important to have reliable and early detection of any break that may have occurred within such a cable, or within any other structure subjected to possible tensile or compressive forces.
It is known practice to detect the onset of breakage within a cable using an acoustic examination of the cable. The energy released as a constituent strand of the cable breaks is thus picked up and recorded using a microphone. However, this technique is able to detect breakage only at the instant at which it occurs. It does not directly provide the history of the number of breaks nor does it give any deterministic indication as to the condition of the cable. Neither is it able to characterize the breaks that have occurred directly, particularly in terms of their location and their extent.
It is a first object of the present invention to alleviate the disadvantages of the known art.
One object of the invention is more specifically to detect breaks that may have occurred within a structure such as a cable.
Another object of the invention is to characterize the breaks, particularly in terms of their location and extent.